Medium Voltage Application Guide

14044.A
SCHEMATIC DIAGRAMS
5.4 Automatic Changeover Systems
In medium voltage distribution networks, it is important to maintain a high level of reliability. Industry relies on the
continuous operation of critical plant, which requires no or little disruption to the electrical supply. There are
various methods for building redundancy of supply into a system. The most commonly used methods are a dual
transformer fed supply and/or a standby generator sized to keep critical plant online.
Automatic changeover systems are designed to monitor and maintain continuous supply. Today's technology
allows the supervision and control of an entire distribution system from a single controller, referred to as an
automatic transfer switch (ATS) or automatic changeover unit (ACU). Although there are a wide range of
products available, they all have the same primary function.
The equipment monitors 3-phase voltages on all power sources. Power supply sources are often prioritised and if
any phase voltage on the primary power source falls outside a predetermined range for a specific amount of time,
the power source is switched over to a back-up supply. Once the primary power source is re-established, the
power source is switched back to the primary power source. In medium voltage systems, switching is performed
using either contactors or circuit breakers. Most ATS controllers can control both types of switching devices.
Automatic changeover systems range from simple main/standby set-ups to highly complex distribution systems and
this is where the selection of an appropriate ATS controller is important. Systems can be operated in automatic or
manual mode.
In AUTO mode, supervision and power source switching is carried out entirely by the ATS controller.
In MANUAL mode, power source switching is carried out by manual selection via the ATS controller.
NOTE
For safety reasons, manual mode cannot be used in certain network configurations.
Overview
The following are examples of common operating modes in automatic changeover systems. Most ATS controllers
can be programmed to operate in any one of these modes.
N1+N2
N1
N2
K1
K2
AUTO mode with line priority:
N1 is the prioritised power source and, if healthy, will always supply the receiving network (DB). If N1 is lost, the
controller switches over to power source N2. The controller switches back to N1 once it has been re-established.
AUTO mode without line priority:
The first power source verified as healthy will supply the receiving network (DB). If this power source is lost, the
other power source will be selected and remain as the supply as long as it is healthy. If both power sources are lost,
both sources N1 and N2 are isolated from the receiving network (DB)
MANUAL mode:
DB
Select N1 or N2 as the power source
710-12280-00A Medium Voltage Application Guide Page 175